The present invention is directed to the use of metal oxides and salts in combination with anhydride/acid modified polymers to enhance adhesion of thermoplastic elastomers to steels such as stainless steel and cold-rolled steel.
Polyolefins and thermoplastic elastomers rich in polyolefins have traditionally had poor adhesion to metal surfaces, including textile fibers surfaces. Experiments with thermoplastic vulcanizates (TPV), a subset of thermoplastic elastomers (TPE), using formulations similar to those of U.S. Pat. Nos. 4,130,534 and 4,130,535 resulted in peel strengths of only 0.5 to 1.0 pounds per linear inch (pli) between the TPV and textile fibers after melt processing the TPV to the textile fibers. The industrial hose and belting markets generally require a peel strength of at least 8 to 12 pli for candidate matrix materials for fiber reinforced hoses and belting. While polyolefins and TPVs from polyolefins have benefits over plasticized polyvinyl chloride resin (in terms of chemical resistance and physical properties after aging) and over crosslinked rubbers (in terms of processability and physical properties after aging) their use has been limited due to low adhesion values (low peel strengths).
Metal oxides such as ZnO have been incorporated into rubber and TPE blends in low amounts, usually 2.0 parts or less by weight based upon 100 parts by weight of the rubber, but such use does not significantly promote the bonding of the TPE to metals. An article by G. T. Carpenter in Rubber Chemistry and Technology, Vol. 51 (1978), pages 788-798, entitled “The Effect of Zinc Oxide Particle Size and Shape on Adhesion of Rubber to Brass-Coated Steel Radial Tire Cord”, reported the use of ZnO in amounts ranging from 2 to 11 phr, but the substrates are brass, and so they must be metals that contain metallic zinc, and the bonding process is related to thermoset rubber. Bonding between substrates and thermoset rubbers occurs during the curing process, during which chemical crosslinks are formed between the substrates and the rubber. On the other hand, for bonding between substrates and thermoplastic elastomers, the cure either has completed before the bonding process, such is the case for TPV's, or does not occur at all in the case of physically crosslinked TPE's.
Other teachings, such as International publication WO 01/79371 to Kim, suggest the use of zinc diacrylate or dimethacrylate to improve the adhesion of thermosetting compositions to hydrophilic metal surfaces, although only copper foil is exemplified. An article “Adhesion of Modified PE/EPDM Blends to Steel”, by Viksne et al, in J.M.S.—Pure Appl. Chem., A35(7&8), pp. 1165-1185 (1998) teaches the use of dicumyl peroxide in the presence of coagent, zinc diacrylate, to increase adhesion against steel, compared to dicumyl peroxide alone. These studies are also related to bonding between thermoset rubber and substrates.
Efforts have been made to improve adhesion. For example, U.S. Pat. No. 4,957,968 to Adur et al. teaches an adhesive thermoplastic elastomer which is a blend of a) at least a polyolefin modified by a chemically reactive functional group, b) at least one other polymer, and c) at least one olefinic elastomer, and which is adherent to metal, as well as glass, wood, polyolefins, and polar polymers with no pretreatment or use of other adhesives. U.S. Pat. No. 6,503,984 to Johnson et al. teaches that combinations of a low flexural modulus and low crystalinity polyolefin and a functionalized polyolefin will result in excellent adhesion to metals and polar polymers, especially fibers there from.